Projection arrangements known from the prior art, such as, for example, DLP projectors, include a color wheel and a discharge lamp for illuminating the color wheel. The lamps are in this case operated using alternating current and driven by a ballast. In the case of commutation of the lamp current, polarity reversal of the two electrodes of the discharge lamp takes place. When matching the operating frequency of the discharge lamp to the color wheel, the ballast provides a suitable lamp current with a specific wave form in accordance with a commutation scheme.
Gas discharge lamps for video projection applications usually consist of a pair of tungsten electrodes, on which small peaks grow in the case of a suitable mode of operation. These peaks act as root point for the discharge arc and are essential for good performance of the lamp, in particular in respect of a high luminance, low tendency to flicker and low burnback tendency.
For a stable lamp performance, both the geometry and the position of the peaks on the electrode tip need to be kept as constant as possible over the lamp life. The electrode peaks achieve temperatures in the vicinity of the melting point of tungsten during operation at their frontmost end, with the result that tungsten evaporates perpetually. Correspondingly, material needs to be subsequently delivered out of the electrode tip continuously. This can be achieved by virtue of the zone of molten tungsten in the electrode peak being varied cyclically in terms of its extent by temperature modulation. The melting and solidifying processes occurring in the process, in interaction with the high surface tension of tungsten, effect material transport out of the electrode tip forwards into the electrode peak.
Furthermore, the mode of lamp operation needs to be closely matched to the customer application. In particular in DLP projectors, precise synchronization with the color wheel generally used therein needs to take place.
It is known from the prior art that cyclic melting of the peak and an accompanying growth of the peaks can be achieved with the aid of a so-called maintenance pulse at the end of each current half-cycle directly prior to the commutation, as described, for example, in EP 766906 B1. Furthermore, U.S. Pat. No. 7,994,734 B2 and DE 10 2009 006 338 A1 disclose using DC phases repeatedly for avoiding excessive growth of peaks and for reshaping of the peaks. These DC phases are used in this case depending on the lamp voltage. Since the lamp voltage increases proportionally to the spacing between the electrode peaks, it is therefore possible to use the lamp voltage to draw a conclusion on the spacing between the peaks.
U.S. Pat. No. 7,994,734 B2 in this case is concerned with the regression of electrodes since excessive electrode growth results in flicker phenomena and an excessively high lamp current. Since the lamp voltage, as has been mentioned, gives some indication of the spacing between the electrode peaks, the operation of the lamp is regulated depending on the measured lamp voltage as well in accordance with U.S. Pat. No. 7,994,734 B2. If the lamp voltage in this case falls below a limit value, commutations are suppressed in the commutation scheme of the lamp current with which the lamp is operated, with the result that DC phases likewise result here. As a result, fusing-off of the electrode peaks and therefore regression thereof are effected. In this case, DC phases are set with intervening periods which are typically greater than 150 seconds. By virtue of this measure, however, only excessive peak growth can be avoided, but stabilization of the peak position is not possible thereby.
In accordance with DE 10 2009 006 338 A1, a check is performed during operation of the lamp to ascertain whether the lamp voltage is lower than a lower limit value, greater than an upper limit value or between these two limit values. Depending on the range in which the lamp voltage is, DC voltage phases are applied repeatedly with an intervening time period with a duration which depends on the measured lamp voltage. The intervening time period is in this case between 180 s and 900 s in order not to subject the electrodes of the lamp to too much loading. Very long DC voltage phases in this case melt the entire end of the electrode for a short period of time, the electrode ends form to give a spherical shape owing to the surface tension, and therefore the regression of the electrode peaks is effected. Short DC voltage phases only effect fusing-over of the electrode peaks, with the result that the shape of the electrode peaks can be influenced. In order to promote the growth of the peaks, after a long DC phase, a maintenance pulse (already mentioned above) is used. By virtue of the application of these measures, the spacing between the electrode peaks can be influenced and fissuring of the electrode peaks can be avoided, depending on the lamp voltage. However, by virtue of this method, it is likewise not possible to achieve sufficient stabilization of the peak position since in particular even non-fissured electrode peaks can migrate from the center over the course of the life of the lamp, which therefore shortens the life of the lamp.
Therefore, with this procedure only the size of the peak but not the position of the peak can be stabilized sufficiently well. With continuing reduction in size of the effective aperture in modern day projectors, shifting of the peaks is no longer tolerable, however, since a change in the peak position results in a massive reduction in the coupling-in efficiency of the light into the projector optical element and therefore a premature end of life.
One approach for solving this problem consists in modulating the frequency of the lamp current (waveform) with which the lamp is operated in terms of time, as described, for example, in WO 2013092750 A1. In this case, the advantage consists in the well-metered fusing of the electrode peaks, which firstly enables sufficient growth, but secondly also enables stabilization of the peak position. This effect is generally achieved most effectively with waveforms having an average frequency in the region of 90 Hz.
One disadvantage of this solution, however, consists in flicker phenomena, so-called scintillations, which are clearly perceivable on the projection screen. These flicker phenomena can only be combated, as matters currently stand, by virtue of using either waveforms with a symmetrical frequency of 60 Hz or else frequency-modulated, asymmetrical waveforms with much higher frequencies. With both variants, flicker-free lamp operation can be realized, but at the cost of severely reduced life performance.
A further disadvantage of this solution has proven to be the fact that it is often difficult in the case of a specific customer application, owing to the rigidly preset color wheel, to find a suitable waveform with advantageous commutation schemes. In addition, predictions of the response of the lamp and its electrode peaks for a specific commutation scheme can only be made with difficulty or not at all. In order to check whether a specific commutation scheme is suitable, i.e. meets specific criteria in respect of the formation of the electrode peaks and therefore in respect of the life of the lamp, it is necessary to operate a lamp with a lamp current in accordance with this commutation scheme at least for a large proportion of its life or even for its entire life. This is extremely time-consuming and therefore makes the search for suitable commutation schemes and operating modes for a discharge lamp more difficult. A mode of operation of a discharge lamp which reconciles the two requirements for a long life of the discharge lamp and flicker-free operation of the discharge lamp to a satisfactory extent has not been found as yet, however.